1. Combine the biscuit crumbs and melted butter in a bowl; mix well.  Press the mixture into base and sides of a 23 cm round flan tin.  Refrigerate for 20 minutes or until firm.

2. White Chocolate Cream: Place white chocolate and cream in a small heatproof bowl, stir over simmering water until chocolate melts and mixture is smooth.  Spread over base of prepared flan.  Refrigerate until set.

3. Mocha Mousse: Combine chocolate, melted butter, cream, egg yolks, and water in a large bowl; mix well.  Using electric beaters, whip cream until firm peaks form; fold through chocolate mixture.  Spread into prepared flan case, refrigerate until firm.

4. Topping: Just before serving, whip cream until soft peaks form, spread over mousse and dust lightly with sifted cocoa.